The present invention pertains to the perforation of cased boreholes and more particularly to individually perforating different formations or twice perforating the same formation in a single trip into the well.
After a wellbore has been formed into the ground and the casing has been cemented into place, the hydrocarbon-containing formation usually is communicated with the casing interior by forming a plurality of perforations through the casing which extend radially away from the casing and out into the formation, thereby communicating the hydrocarbon-containing formation with the interior of the casing.
It is now common practice to run a jet perforating gun downhole on a pipe string and to fire the gun by the employment of a gun firing head which is actuated by a bar dropped down through the interior of the pipe string. Completion techniques involving this known completion process are described in U.S. Pat. Nos. 3,706,344 and 4,009,757.
Many times it is desirable to perforate more than one formation located in the well. One prior method is to run a jet perforating gun downhole on a pipe string to a first formation, perforate the first formation, pull out the pipe string and expended gun, replace the used gun with a new perforating gun, run the new gun down to a second formation and perforate the second formation. This method requires two trips into the well.
Another method is illustrated in FIG. 1 of the drawings. A pipe string supports an upper and a lower string of perforating guns. A lower pipe string of a predetermined length and made up of individual pipe lengths, is disposed between the two strings of perforating guns whereby upon lowering the guns downhole, the upper and lower strings of guns are located adjacent the upper and lower formations, respectively, to be perforated in the well. A prima cord extends from the upper guns, through the lower pipe string, to the lower string of guns. Booster caps are required at the joints of the pipe lengths making up the lower pipe string to permit connection of the pipe lengths and the prima cord. The connections of the individual pipe lengths must be hermetically sealed since the prima cord would be rendered inoperative if the prima cord was not completely dry. One of the principal deficiencies of this method is that a trained crew must be used to assemble and lower the prima cord with booster caps and the hermetically sealed lower pipe string into the well. The requirement of hermetically sealed pipe precludes the use of pipe normally on hand at the drill site and special pipe must be brought to the drill site for this purpose. Many times the distance between the two strings of guns is several hundred feet. Further, this method has the deficiency in that the two formations must be perforated simultaneously since the strings of guns will be detonated together.
There are several problems with both of these methods. Drilling oil wells is expensive and each round trip adds to that cost. Another problem is that formations are usually far apart, often in the range of 600 to 900 feet, so providing hermetically sealed pipe is very expensive. The prima cord is also expensive. It is further desirable to be able to perforate the two formations independently so that the first formation can be perforated and then the other formation perforated at a later time.
The number of perforations per foot of formation adjacent the cased borehole is limited in a single trip into the well by the number of charges which can be disposed within a perforating gun. Charges must be of a particular size to contain sufficient explosive for the cased borehole to be perforated. Many times it is desirable to increase the number of perforations per foot of formation such as where a limited length of the cased borehole passes through the formation. Oftentimes in the past it has been necessary to lower a first string of guns into the well to perforate the formation and then, after removing the first string of guns, lower a second string of guns adjacent the formation of perforate the formation again thereby increasing the number of perforations per foot.
The present method overcomes the deficiencies of the prior art.